Summary of work: Measurements of regional cerebral blood flow (rCBF) during cognitive stimulation, and the effect of drugs which modulate synaptic function, can be used to examine synaptic integrity of the brain in health and disease. In healthy subjects performing an active working memory for faces task, rCBF effects of physostigmine, an inhibitor of acetylcholinesterase, were measured by positron emission tomography with [15-O]-water as a tracer. Significant effects became stable within 40 minutes after establishing a steady-state plasma concentration of physostigmine. No habituation with repeated testing was evident. Subjects performing the task had increased rCBF in occipitotemporal visual brain regions and in right prefrontal cortex. Physostigmine reduced rCBF increments in the right prefrontal cortex in relation to a decrease in task reaction time, indicating that the right prefrontal cortex is involved in effort and subject to cholinergic modulation. Reaction time also correlated with changes in rCBF in inferior temporal cortex, left hippocampus, anterior cingulate and primary visual cortex, suggesting that a network of brain regions that are involved in task performance are affected by physostigmine as a group of regions. Whereas young and old subjects showed differences in resting-state rCBF, while performing the working memory task during physostigmine infusion, both groups showed similar reductions in reaction time.